Removable disk cartridges have been available on the market for some time. Unlike fixed disk drive systems, removable disk cartridge systems enable a user to easily replace a relatively high capacity disk, allowing for convenient exchange of large amounts of information between remote sites and for greatly increased system storage capacity.
Removable disk cartridge technology continues to advance, providing the user with cartridges and disk drives of increasing performance and data storage capacity. These advancements are universally beneficial, leading to less cost per unit of data stored and enhanced accuracy of data storage and retrieval operations. Nevertheless, problems associated with technological advancement do occur. One of the most critical problems in the area of removable cartridge technology concerns downward compatibility.
Because removable disk cartridges are by definition removable, they can be used interchangeably between one disk drive product and another. Thus, a removable cartridge originally designed for an older, lower-capacity disk drive can often be inserted in a newer, higher-capacity disk drive and data can be written on or read from the disk in the lower-capacity cartridge by the read/write head in the higher-capacity drive. However, after the higher-capacity drive writes on a lower-capacity cartridge, difficulties may occur when the re-written lower-capacity cartridge is reinserted back into the older, lower-capacity disk drive. One reason for this difficulty is that the read/write head width and associated track pitch of the higher-capacity disk drive are usually smaller than the head width and associated track pitch of the lower-capacity disk drive. As a result, portions of the old data signals recorded in a given track by the lower-capacity disk drive remain in "sidebands" on either side of the data newly recorded in the higher-capacity disk drive, giving rise to a potential for interference when the lower-capacity disk drive performs read operations.
The sideband phenomenon is illustrated in FIG. 1A, which depicts an A/B servo pattern recorded on a lower-capacity removable disk cartridge over a localized track region. As can be seen from FIG. 1A, the recording of data in a data track 2 of a removable cartridge originally designed for a lower-capacity disk drive, using a read/write head 4 of a higher-capacity disk drive, leaves inner and outer sidebands 6, 8 containing portions of the old data signals recorded in the data track by the (wider) read/write head (not shown) of the lower-capacity disk drive. If the removable cartridge with the new data recorded on it is subsequently removed from the higher-capacity disk drive and reinserted into the lower-capacity disk drive, these inner and outer sidebands will create interference during the read operation in the lower-capacity drive. Stray or random flux reversals in the sidebands will impact on the lower-density read/write head as it passes over the data recorded by the high-density head, leading to spurious or corrupted data readings.
Techniques have been developed to eliminate sideband interference in lower-capacity removable cartridges containing data re-recorded with a higher-density read/write head. For example, the SyQuest Model SQ5110C disk drive manufactured by the assignee of the present invention can accept 88 Megabyte removable cartridges designed for the SQ5110C drive and 44 Megabyte cartridges designed for older SyQuest disk drive products. When new data is to be stored on the disk of a lower-capacity 44 MB cartridge using the higher-density SQ5110C read/write head, the SQ5110C drive first performs a DC erase of the old data originally recorded on the 44 MB disk by the old, lower-density read/write head. As shown in FIGS. 2A-2B, erasing is accomplished by injecting a "static" or DC offset signal into the track following feedback loop of the SQ5110C drive so as to reposition the smaller, higher-density SQ5110C head over the inner and outer sidebands of each data track during erase operations.
The "static" offset injection of the SQ5110C disk drive works well as long as the ratio between the higher-density head width and the lower-density track pitch is relatively small. However, if the higher-density head width becomes too small relative to the lower-density track pitch, errors can arise from the servo operation performed by the track following feedback loop in the disk drive, depending upon the servo technique utilized for track following. To understand how this happens, further reference is had to FIG. 1A.
Many removable cartridge disk drive products employ the well-known embedded A/B servo burst scheme for track following. FIG. 1A shows an A/B type servo pattern followed by a data field recorded on the surface of the disk in the lower-capacity cartridge. Due to the radial displacement of the "A" burst relative to the "B" burst in a given servo sector, the "A" and "B" bursts are displaced on either side of the track centerline. When the head is positioned exactly over track centerline, approximately one-half of the "A" burst will be read followed by one-half of the "B" burst in a time displaced fashion. As the head moves off the centerline of a track, the amplitude of one burst decreases while the amplitude of the other burst increases depending on the direction of misalignment. In this manner, a position error signal can be derived from the relative amplitudes of the bursts by rectifying and peak detecting the readout from the head as it passes over the "A" and "B" bursts, and determining the difference in amplitude between the bursts.
Where the width of the higher-density read/write head 4 is significantly less than the pitch of lower-density track 2, an A/B burst amplitude reading of the type depicted in FIG. 1B will be produced. FIG. 1C illustrates a typical A/B servo feedback waveform derived from the A/B burst amplitude pattern of FIG. 1B. As can be observed in both FIGS. 1B and 1C, the relatively small head width of higher-density read/write head 4 produces saturation regions 10 in the A/B burst amplitude signal and A/B servo feedback waveform generated by the track following circuitry. These saturation regions contain no useful servo information, inasmuch as any shifting of the read/write head relative to track centerline which occurs while the head is in the saturation region produces no change in the A/B servo feedback waveform and therefore cannot be detected.
The difficulties encountered in using the prior art static offset technique to eliminate sideband interference when a higher-density head writes data over a lower-density data track become more apparent upon consideration of FIG. 2C. As can be seen in FIG. 2C, application of the static offset to eliminate the sideband on either side of track centerline produces a constant state offset condition wherein the higher-density head is located along a region of the A/B servo feedback waveform unacceptably near the saturation region of the waveform. Small deviations from the optimum head offset position during the sideband erasing operation can move the head into the saturation region, resulting in no useful feedback and, in effect, loss of servo capability.
A solution to the described interference problem is accordingly necessary if true downward compatibility for older cartridges recorded in lower-capacity disk drives is to be achieved in higher-capacity disk drives.
Another problem in the art is the difficulty of the disk drive to determine initially which capacity of disk has been inserted into the drive. This problem is compounded by the fact that track density, bit density and control format may vary from cartridge to cartridge depending on storage capacity. The disk drive may not be able to read or write information until it successfully identifies the type of cartridge that has been loaded. One technique developed to enable disk drives to distinguish between disks is the placement of a mechanical feature (such as a slot) on the disk and the use of a dedicated sensor to read and interpret the mechanical feature. However, the use of a dedicated sensor requires extra components and prevents the use of a uniform physical cartridge for all storage capacities. It would therefore be advantageous to have a disk drive system capable of distinguishing between cartridges of different storage capacities without requiring the use of a dedicated sensor in the disk drive or a mechanical feature on the cartridge, and which can distinguish cartridges rapidly so as not to cause annoying delay to the user.